Notice: There is no legacy documentation available for this item, so you are seeing the current documentation.
When you are setting up the account your logo size should be 256X256px.
This is available under Settings > Site Settings > Installation Settings > Update Site Logo

For adding creatives and making your notification more interesting, you can add Notification Large Image.
The size used as per the devices are mentioned below –
- For Desktop – 364X180 px
- For Mobile – 364X80 px
Please note the large image only appears for the Chrome browser (Version 57 and above). It does not appear for Safari/Firefox.